Manufacturing method of high-strength and high-toughness thin steel and heat treatment apparatus

ABSTRACT

To provide a technique suitable for elevating strength and toughness of a thin low-carbon steel. By performing rapid heating and rapid cooling to a thin low-carbon steel which is an ordinary steel with a thickness of 1.2 mm or less, a steel where a microstructure becomes a duplex grain size structure mixed with crystal grains having different grain diameters, which is not homogeneous, preferably, hard phase structures are contained in addition to the duplex grain size structure is obtained, and a high-strength and high-toughness thin low-carbon steel is obtained. Further, by performing a heat treatment process involving rapid heating and rapid cooling multiple times, a duplex grain size structure of crystal grains with smaller grain diameters or a hard phase structure contained therein is obtained, so that a thin low-carbon steel with higher strength and higher toughness is obtained.

CROSS REFERENCE

This application is a division of and is based upon and claims thebenefit of priority under 35 U.S.C. §120 for U.S. Ser. No. 13/202,991,filed Oct. 28, 2011, the entire contents of which is incorporated hereinby reference. U.S. Ser. No. 13/202,991 is a National Stage of PCTJP10/052,651, filed Feb. 22, 2010, and claims the benefit of priorityunder 35 U.S.C. §119 from Japanese Patent Application No. 2009-041571,filed Feb. 24, 2009.

TECHNICAL FIELD

The present invention relates to a manufacturing method of ahigh-strength and high-toughness thin steel which performs heattreatment to a thin low-carbon steel as a reception material tomanufacture a high-strength and high-toughness thin steel, and a heattreatment apparatus.

BACKGROUND ART

For example, a seat frame for transportation equipment such as anautomobile or an airplane is strongly required to be reduced in weightin view of fuel consumption improvement, carbon dioxide emission controlor the like, and thus, high strength of a steal material used forforming a seat frame is in demand. On the other hand, the seat frame isalso required to have not only high strength but also high toughness(also including ductility) in view of impact absorbing properties owingto deformation or the like. As techniques satisfying these demands, forexample, high-strength steel plates disclosed in Patent Literatures 1 to3 are known.

Each of the high-strength steel plates disclosed in these Literaturesassumes control on an addition amount of an alloy element other thancarbon, and it is made to contain, for example, Mn, Mo, Cr, or the likein a predetermined amount or more to secure a predetermined hardness orductility. Then, for use as a steel material for an automobile or thelike, the high-strength steel plate is finally cold-rolled down to athickness of 1.2 mm, but heat treatment performed at a step before thecold rolling is to heat-roll a steel slab to a thickness of 3.2 mm. Thatis, since the techniques disclosed in these Literature are techniquesfor obtaining a steel plate with a thickness of several mm or thicker,it is necessary to achieve evenness of a microstructure including aplate-thickness direction in the steel plate in the heat treatment, andthus, control on an addition amount of an alloy element is an importantfactor.

On the other hand, in Patent Literatures 4 to 5, techniques of achievinghigh strength of an ordinary low-carbon steel have been disclosed.Patent Literature 4 discloses a technique proposed in order to solvesuch a problem that, since tempering property of an ordinary low-carbonsteel was poor in the previous technique, when a martensitic state wasutilized as an originating structure, an uneven duplex grain structurewas produced during an annealing time so that a predeterminedhigh-strength and high-ductility steel material could not be obtained.Therefore, in Patent Literature 4, after an ordinary low-carbon steel istempered to achieve martensitic phase of 90% or more, an ultra-finecrystal grain ferrite structure with grain diameters of 1.0 μm or lessis obtained by performing cold-rolling with a total reduction rangingfrom 20% to less than 80% and performing annealing. Patent Literature 5is the technique which has been proposed by the present applicant, wherehigh strength is achieved by performing working process for elevatinginternal stress, such as press forming, and achieving refinement andduplex grain sizing of a metal structure of a low-carbon steel by heattreatment.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent No. 4005517-   Patent Literature 2: JP-A-2005-213640-   Patent Literature 3: JP-A-2008-297609-   Patent Literature 4: Japanese Patent No. 4189133-   Patent Literature 5: JP-A-2008-13835

SUMMARY OF INVENTION Technical Problem

A demand for reduction of cost or recycling efficiency of resources of aseat frame for an automobile or the like increasingly becomes high fromnow due to energy saving, accommodation to environmental problems, orthe like. Therefore, rather than high strength or high toughnessachieved by alloying like the techniques described in Patent Literatures1 to 3, achievement using an ordinary low-carbon steel which elevatesthe recycling efficiency is desired. Further, these techniques aretechniques mainly implemented by iron and steel material manufacturersfor producing a predetermined high-strength and high-toughness steelfrom a steel slab and they are not techniques which can be utilized byprocessing manufacturer which manufactures a seat frame or the likeusing a commercially-available steel. If the processing manufacturerpurchases an ordinary steel (common steel) which is inexpensive and easyto work from an iron and steel material manufacturer rather thanpurchasing a material sold as a high-strength and high-toughness steelby the iron and steel material manufacturer, and can achievehigh-strength and high-toughness at a required portion of the ordinarysteel if necessary, cost reduction of a seat frame can be achieved.

The technique disclosed in Patent Literature 4 is a technique forobtaining desired strength and ductility using an ordinary low-carbonsteel as a reception material of heat treatment, but it requires aprocess where after the whole steel material is martensitized, it iscold-rolled to achieve refinement homogeneously. Therefore, aninstallation provided with a rolling function is required, whichincludes a problem regarding installation cost and manufacturing cost.As apparent from such a fact that an ordinary low-carbon steel materialwith a thickness of 2 mm is exemplified in an Example in patentLiterature 4, in order to achieve high strength and high ductility of asteel with a certain thickness, it is necessary to achieve homogeneousrefinement in a plate-thickness direction so that a cold-rolling stepunder predetermined conditions is essential after the martensitization.

In the case of the technique disclosed in Patent Literature 5,refinement and high strength are achieved by heat-treating a thincold-rolled steel plate and a thin hot-rolled steel plate having athickness of 1.2 mm and a thickness of 1.0 mm in Examples, but there isroom for further improvement of toughness.

The present invention has been made in view of the above circumstances,and a problem to be solved thereof is to provide a technique excellentin cost reduction and recyclability as well as suitable for elevatingstrength and toughness of a thin low-carbon steel which is an ordinarysteel on the side of a processing manufacturer.

Solution to Problem

As a result of the present inventors' keen study for solving the aboveproblem, since a thin low-carbon steel which is an ordinary steel(common steel) with a thickness of 1.2 mm or less is thin, it has a highheat capacity and it tends to be rapidly heated and rapidly cooledeasily. The present inventors have focused on such a fact that, owing toa duplex grain size structure where crystal grains different in graindiameter and formed by rapid heating and rapid cooling have been mixed,preferably, owing to a structure where hard phase structure higher inhardness than such a duplex grain size structure has been contained inaddition to the duplex grain size structure, a steel where strength andtoughness have been balanced in high level can be obtained even ifrefined crystal grains with a size of 1 μm or less are not contained ata high percentage as in a thick steel or even unless homogenization isachieved in a plate-thickness direction. The present inventors have alsofocused on such a fact that it is effective to perform a heat treatmentprocess involving rapid heating and rapid cooling multiple times withoutrequiring a cold-rolling step after heat treatment or the like in orderto obtain a duplex grain size structure where grain diameters of crystalgrains are different in this manner in a thin low-carbon steel.

That is, a manufacturing method of a high-strength and high-toughnessthin steel according to the present invention is a method formanufacturing a high-strength and high-toughness thin steel byheat-treating a steel raw material, comprising: using a thin low-carbonsteel which is an ordinary steel worked to have a thickness of 1.2 mm orless as the steel raw material which is a reception material of heattreatment; a first process of rapidly cooling the thin low-carbon steelafter rapid heating thereof to obtain a martensite structure; and asecond process of rapidly cooling the thin low-carbon steel which havebeen subjected to the first process after rapidly reheating the same toa temperature lower than a temperature at a rapid heating time in thefirst process, wherein the first process and the second process areimplemented while the thin low-carbon steel is being relatively moved toeach heating section and each cooling section which perform rapidheating and rapid cooling treatments in the first process and the secondprocess; the first process includes a step of rapidly heating the thinlow-carbon steel up to a temperature of 1000° C. or higher at a rate of300° C./second or more and a step of rapidly cooling the thin low-carbonsteel at a rate of 300° C./second or more after the thin low-carbonsteel is held at a temperature of 900° C. or higher within ten seconds;and the second process includes a step of rapidly heating the thinlow-carbon steel up to a temperature of 700° C. or higher at a rate of300° C./second or more after the cooling in the first process and a stepof rapidly cooling the thin low-carbon steel at a rate of 300° C./secondor more after the thin low-carbon steel is held at a temperature of 600°C. or higher within ten seconds.

It is preferred that the rapid heating in the first process and therapid heating in the second process are implemented by high-frequencyinduction heating. Further, the rapid heating in the first process andthe rapid heating in the second process can be implemented by laserheating. The treatments in the first process and the second process canalso be performed multiple times.

It is preferred that C content of the thin low-carbon steel is in arange of 0.01 to 0.12% by mass % and the rest thereof is composed ofiron and inevitable impurities. It is preferred that rapid heating isperformed up to a temperature in a range of 1000° C. to 1250° C. at therapid heating step in the first process and rapid heating is performedup to a temperature in a range of 750° C. to 1050° C. at the rapidheating step in the second process. It is preferred that a holding timebefore the rapid cooling after the rapid heating in the first process isset within five seconds and the holding time before the rapid coolingafter the rapid heating in the second process is set within fiveseconds.

It is preferred that a heat treatment apparatus which treats the thinlow-carbon steel is provided with a first heating section which performsthe rapid heating treatment in the first process, a first coolingsection which performs the rapid cooling treatment in the first process,a second heating section which performs the rapid heating treatment inthe second process, and a second cooling section which performs therapid cooling treatment in the second process, and the thin low-carbonsteel is sequentially treated in the first heating section, the firstcooling section, the second heating section, and the second coolingsection. Further, such a configuration can be adopted that the firstheating section and the second heating section are composed of oneheating section having a predetermined length extending in a movingdirection, and the cooling treatment in the first process and thecooling treatment in the second process can be performed to the thinlow-carbon steel which is a target to be treated from opposite facesthereof.

It is preferred that, when the thin low-carbon steel is pipe-shaped,treatment is performed while the thin low-carbon steel is being rotated.Further, the steel raw material which is a reception material of heattreatment is preferably a thin low-carbon steel with a thickness of 1.0mm or less, the steel raw material which is a reception material of heattreatment is more preferably a thin low-carbon steel with a thickness of0.8 mm or less, and the steel raw material which is a reception materialof heat treatment is further preferably a thin low-carbon steel with athickness of 0.5 mm or less.

A heat treatment apparatus according to the present invention is a heattreatment apparatus which uses a thin low-carbon steel which is anordinary steel which has been worked to have a thickness of 1.2 mm orless as the steel raw material which is a reception material of heattreatment and which is used to manufacture a high-strength andhigh-toughness thin steel by a first process of rapidly cooling the thinlow-carbon steel after rapid heating thereof to obtain a martensitestructure and a second process of rapidly cooling the thin low-carbonsteel which have been subjected to the first process after rapidlyreheating the same down to a temperature lower than a temperature at arapid heating time in the first process, wherein a work supportingsection which supports the thin low-carbon steel which is a target to betreated, a first heating section which performs the rapid heatingtreatment in the first process, a first cooling section which performsthe rapid cooling treatment in the first process, a second heatingsection which performs the rapid heating treatment in the secondprocess, and a second cooling section which performs the rapid coolingtreatment in the second process are sequentially arranged; and the firstheating section, the first cooling section, the second heating section,and the second cooling section are provided so as to be movable relativeto the work supporting section.

Further, the heat treatment apparatus according to the present inventionis configured such that one heating section having a predeterminedlength extending in a moving direction, the first cooling sectionarranged on the side opposite to the heating section via the work, andthe second cooling section arranged on the same side as the heatingsection or on the same side as the first cooling section via the work tobe separated from the heating section or the first cooling section by apredetermined distance rearward in the moving direction are provided,and the heating section has such a length that a vicinity of a frontportion thereof corresponds to the first cooling section and a vicinityof a rear portion thereof extends rearward in the moving directionbeyond the first cooling section; and the heating section is configuredto have two functions such that the vicinity of the front portion of theheating section has a function of the first heating section whichperforms the rapid heating in the first process and the vicinity of therear portion of the heating section has a function of the second heatingsection which performs the rapid heating in the second process. In thiscase, it is preferred that the second cooling section is disposed on thesame side as the heating section.

Further, such a configuration can be adopted that the work supportingsection is rotatably provided in a supporting state of the thinlow-carbon steel. Furthermore, it is preferred that such a configurationis adopted that each of the heating sections includes a coil performinghigh-frequency induction heating, and such a configuration can beadopted that each of the heating sections is provided with a laserperforming laser heating.

Advantageous Effect of the Invention

According to the manufacturing method of a high-strength andhigh-toughness thin steel and the heat treatment apparatus, byperforming rapid heating and rapid cooling to a thin low-carbon steelwhich is an ordinary steel with a thickness of 1.2 mm or less, a duplexgrain size structure where a microstructure is inhomogeneous and crystalgrains different in grain diameter have been mixed is obtained,preferably, a structure where a hard phase structure is contained inaddition to the duplex grain size structure is obtained, so that a thinlow-carbon steel with high strength and high toughness is obtained.Further, by performing a heat treatment process involving rapid heatingand rapid cooling multiple times, a duplex grain size structure ofcrystal grains having smaller grain diameters or a hard phase structurecontained therein is obtained, so that a thin low-carbon steel withhigher strength and higher toughness is obtained. In addition, by usinga heat treatment apparatus provided with two heating sections and twocooling sections arranged in a predetermined order, the plurality oftimes of the rapid heating and the rapid cooling can be efficientlyperformed. Furthermore, by using one heating section having apredetermined length and arranging the first cooling section on the sideopposite to the heating section via a work, an apparatus can be madesimpler, which results in contribution to manufacturing cost reductionof a high-strength and high-toughness thin steel.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram showing one example of a schematic configuration ofa high-frequency induction heating apparatus, FIG. 1B is a diagramshowing a schematic configuration of a preferred example of thehigh-frequency induction heating apparatus, and FIG. 1C is a diagramshowing a schematic configuration of a high-frequency induction heatingapparatus where one heating section which performs rapid heating in afirst process and in a second process is provided and rapid coolingtreatment is performed from both sides of a work;

FIG. 2 is a graph showing temperature conditions of treatment conditions(A) and (B) in Test Example 1;

FIGS. 3A to 3C are electron microscope photographs of microstructures ofSamples 1 to 3 which were treated under the treatment conditions (A) and(B) of Test Example 1;

FIG. 4 is a graph showing a temperature condition of a treatmentcondition (C) in Test Example 2;

FIG. 5 is an electron microscope photograph of a microstructure ofSample 1 which was treated under the treatment condition (C) of TestExample 2;

FIG. 6A is electron microscope photographs of microstructures of rawmaterial states of Sample 1 and Sample 2, and FIGS. 6B and 6C areelectron microscope photographs of respective microstructures of Sample1 (Comparative Sample 1) and Sample 2(Comparative Sample 2) which weretreated in Comparative Example 1;

FIG. 7 is a graph showing a relationship between hardness (Hv) andfractal dimension of Sample 1 to Sample 3 which were treated by TestExample 1, Test Example 2, and Comparative Example 1;

FIG. 8 is a graph showing a relationship between breaking elongation andfractal dimension of Sample 1 to Sample 2 which were treated by TestExample 1, Test Example 2, and Comparative Example 1;

FIGS. 9A and 9B are diagrams for explaining a measuring method of abending test of Test Example 3;

FIG. 10 is a graph showing a measurement result of the bending test ofTest Example 3;

FIG. 11 is a graph showing a measurement result of a tensile test ofTest Example 4; and

FIG. 12 is a graph showing a measurement result of a tensile test of apipe-shaped steel of Test Example 5.

DESCRIPTION OF EMBODIMENTS

In a method for manufacturing a high-strength and high-toughness thinsteel according to the present invention, a steel raw material which isa reception material of heat treatment is a commercially-availableordinary steel (common steel), which is a thin and low-carbon one(hereinafter, called “thin low-carbon steel”). As the thin low-carbonsteel, a rolled steel plate which is inexpensive and excellent inworkability and which is used in a seat frame of an automobile or thelike is suitable, the rolled steel plate including both a cold-rolledsteel plate and a hot-rolled steel plate. The thickness of the rolledsteel plate is 1.2 mm or less. When the thickness of the rolled steel isthicker than 1.2 mm, a large heat source and a large-scaled coolinginstallation are required for performing rapid heating and rapid coolingin order to achieve high strength and high toughness, and becausehomogeneity of crystal grains is required in a plate-thicknessdirection, it is difficult to perform control, so that such a thickrolled steel plate is unfit for a steel raw material which is a targetto be treated of the present invention. A steel raw material to betreated of the present invention, which does not involve a rollingprocess and whose high strength and high toughness should be achieved byonly a heat treatment process of rapid heating and rapid cooling ispreferably a thin low-carbon steel with a thickness of 1.0 mm or less,more preferably a thin low-carbon steel with a thickness of 0.8 mm orless, and further preferably a thin low-carbon steel with a thickness of0.5 mm or less.

As the above thin low-carbon steel, a low-carbon steel whose carboncontent is in a range of 0.01 to 0.3% and whose rest is composed of ironand inevitable impurities can be used, but an extremely-low-carbon steelwhose carbon content is in a range of 0.01 to 0.12% and whose rest iscomposed of iron and inevitable impurities is preferably used. By usinga material whose carbon content is lower and is further inexpensive,reduction of a manufacturing cost of a seat frame or the like can beachieved. Further, in the present invention, by performing limitation toa thin material, even if carbon content is low, strength can be elevatedand balance with toughness can be achieved, so that it is unnecessary toperform addition of an alloy element other than carbon and recyclingefficiency is excellent. On the other hand, since there is no limitationabout a component except for the above carbon content, for example, arecycle steel material mixed with a material which was used as anordinary steel, where various components other than carbon were mixed,is also usable. Incidentally, the thin low-carbon steel which is atarget to be treated includes both a plate-shaped one and a pipe-shapedone.

It is preferred that a process of heat-treating the above thinlow-carbon steel is performed according to the following two steps. Thatis, the process of performing heating treatment includes a first processof rapidly cooling a thin low-carbon steel after rapid heating thereofto obtain a martensite structure and a second process of rapidly coolingthe thin low-carbon steel which has been subjected to the first processafter rapidly reheating the thin low-carbon steel up to a temperaturelower than a temperature at the rapid heating time in the first process.Incidentally, it is possible to perform the treatments of the thinlow-carbon steel in the first process and the second process multipletimes in a repeating manner.

The first process includes a step of rapidly heating the thin low-carbonsteel up to a temperature of 1000° C. or higher, preferably, up to atemperature in a range of 1000° C. to 1250° C., at a rate of 300°C./second or more and a step of, after the rapid heating, maintainingthe thin low-carbon steel within ten seconds, preferably, within fiveseconds, until the temperature of the thin low-carbon steel drops to apredetermined temperature of 900° C. or higher, preferably, drops to atemperature in a range of 1000° C. to 1100° C., and thereafter rapidlycooling the thin low-carbon steel at a rate of 300° C./second or more.By rapidly heating the thin low-carbon steel up to the abovetemperature, a metal structure of the thin low-carbon steel isaustenitized and a martensite structure is formed by the rapid cooling,but since the thickness of the thin low-carbon steel which is a targetto be treated of the present invention is 1.2 mm or less, a homogeneousmartensite structure which have escaped relatively coarsening can beformed by, so to speak, ultra-rapid heating and ultra-rapid cooling suchas 300° C./second or more. Incidentally, the rapid heating rate and therapid cooling rate are more preferably set to 500° C./second or more.

The second process includes a step of rapidly heating the thinlow-carbon steel up to 700° C. or higher, preferably, up to atemperature in a range of 750° C. to 1050° C., at a rate of 300°C./second or more, after the cooling in the first process, and a stepof, after the rapid heating, maintaining the thin low-carbon steelwithin ten seconds, preferably, within five seconds, until thetemperature of the thin low-carbon steel drops to a predeterminedtemperature of 600° C. or higher, preferably, to a temperature in arange of 700° C. to 950° C. and thereafter rapidly cooling the thinlow-carbon steel at a rate of 300° C./second or more. When the thinlow-carbon steel which has been subjected to the first process isheat-treated in the second process again, it is preferred that the heattreatment is performed after the temperature of the thin low-carbonsteel drops to 200° C. or lower by the rapid cooling in the firstprocess. The heat treatment in the second process may be performed inanother line at a lower temperature, for example, after the temperaturedrops to room temperature. Incidentally, the rapid heating rate and therapid cooling rate in the second process are more preferably set to 500°C./second or more similarly to the first process.

By performing the above ultra-rapid heating and ultra-rapid cooling inthe second process, the martensite structure is changed, and a duplexgrain size structure which includes a duplex grain size structure ofcrystal grains different in grain diameter in a range from 1 μm to 30 μm(the term “grain diameter” in this text indicates “grain diameter suchas a circular phase”), where crystal grains different in grain diameterhaving an average grain diameter smaller than an average grain diameterof a martensite obtained when heat treatment has been performed in orderto form the martensite, namely, an average grain diameter of amartensite obtained when the heat treatment in the first process hasbeen performed have aggregated, can be finally obtained.

The duplex grain size structure is preferably a structure having aconfiguration where crystal grains with grain diameters of 1 μm to lessthan 5 μm and crystal grains with grain diameters of 5 μm to 30 μm havebeen mixed, and it is further preferably a structure having aconfiguration where crystal grains with grain diameters of 1 μm to lessthan 5 μm and crystal grains with grain diameters of 5 μm to 20 μm havebeen mixed. Since the heat-treated steel has the duplex grain sizestructure different in grain diameter in this manner instead of ahomogeneous grain diameter, a partial elongation occurs in the case ofthe thin low-carbon steel, so that a steel having higher toughness canbe obtained. In order to obtain higher strength, it is preferred thathard phase structures higher in hardness than the duplex grain sizestructure are dispersed in the duplex grain size structure. For example,when the duplex grain size structure is a ferrite structure different ingrain diameter, it is preferred that island-shaped martensites having agrain diameter of 30 μm or less, preferably, 20 μm or less are dispersedin the duplex grain size structure. Thereby, a thin low-carbon steelwith high strength and high toughness where a reaction force due todeflection of a beam due to a bending moment at a transition point froman elastic region to a plastic region is at least 1.5 times that beforeheat treatment in a bending property, a yield point in a tensileproperty has a strength of at least 1.5 times that before heattreatment, and a breaking elongation is at least 1.5 times that in astate where heat treatment forming martensite has been performed in athin low-carbon steel, namely, when the heat treatment in the firstprocess has been performed can be obtained.

In the high-strength and high-toughness thin steel obtained by thepresent invention, a microstructure is the duplex grain size structureof crystal grains different in grain diameter, as described the above,preferably, a structure where hard phase structures such as martensitehave been dispersed in the duplex grain size structure. In the presentinvention, the thin low-carbon steel provided with high strength andhigh toughness is obtained by such a structure control, but the presentinventors have found that the microstructure can be regulated from theview point of fractal dimension of a grain diameter. Though described indetail later, the microstructure of the thin low-carbon steel which hasbeen controlled by the heat treatment like the present invention has afractal dimension of a grain diameter higher than that of a graindiameter in martensite obtained when heat treatment for formingmartensite has been performed, namely, by only the heat treatment in thefirst process.

Incidentally, the term “fractal dimension” is a measure representing thedegree of complexity, where in a figure having a self-similarity, whenthe figure is composed of m similar figures obtained by reducing thefigure into a size of 1/n thereof, fractal dimension (similaritydimension) D is expressed by D=log (m)/log (n)=log (the number ofsimilar figures to an original figure)/log (the number of equaldivisions). Accordingly, the “fractal dimension of a grain diameter” inthis text becomes higher according to advance to further refinement ofcrystal grains.

It is preferred that, as a heat treatment apparatus which performs eachheat treatment in the first process and the second process, ahigh-frequency induction heating apparatus is used. Further, it ispreferred that a heating section (a coil configuring an inductionheating section in the case of an induction heating apparatus) and acooling section (a cooling water supplying section supplying coolingwater) of the high-frequency induction heating apparatus move at apredetermined speed relative to the thin low-carbon steel which is atarget to be heat-treated and the work supporting section. Thereby, therapid heating and the rapid cooling treatment in the above-describedextremely short time can be realized even by a small-scaledinstallation. A moving speed of the heating section (a coil configuringan induction heating section in the case of an induction heatingapparatus) and the cooling section of the high-frequency inductionheating apparatus is preferably set in a range within 30 mm/second, morepreferably set in a range within 18 mm/second. Incidentally, a work(thin low-carbon steel) is supported by the work supporting section, andwhen the work is a plate-shaped one, the work supporting section may beconfigured with a flat plate-shaped table on which the plate-shaped workcan be placed or a grasping section (see FIG. 1A to 1C) which grasps anend portion of the work. Further, when the work is pipe-shaped, it ispreferred that treatment is performed while the work is being rotated,so that it is preferred that such a configuration is adopted that thework supporting section has a grasping section which can grasp thepipe-shape one and the grasping section is rotatable.

As shown in FIG. 1A, as the high-frequency induction heating apparatus,one provided with a heating section and a cooling water supplyingsection in this order can be used. The heating section and the coolingwater supplying section are provided by only one set thereof, where whenthe treatment in the first process is performed, the heating section iscontrolled to a predetermined temperature so that the heating section ismade to function as a first heating section (coil) to perform treatment,and the cooling water supplying section is similarly made to function asa first cooling section (a cooling water supplying section). After thetreatment of the first process is performed, the treatment in the secondprocess is performed again by the high-frequency induction heatingapparatus shown in FIG. 1A. In this case, the heating section iscontrolled to a temperature lower than that in the treatment performedin the first process to be made to function as a second heating section(coil), while the cooling section is made to function as a secondcooling section, so that the treatment is performed. Incidentally, thehigh-frequency induction heating apparatus is not limited to oneprovided with only one set of a heating section and a cooling watersupplying section, but such a configuration is preferably adopted that afirst heating section (coil) and a first cooling section (first coolingwater supplying section) which perform the treatment in the firstprocess and a second heating section (coil) and a second cooling section(second cooling water supplying section) which perform the treatment inthe second section are provided in this order, as shown in FIG. 1B.According to the apparatus shown in FIG. 1B, the first process and thesecond process can be performed continuously, so that a treatment rateof a work is improved.

Further, as shown in FIG. 1C, such a configuration can be adopted thatboth functions of the first heating section in the first process and thesecond heating section in the second process are satisfied by using aheating section (coil) having a predetermined length or longer in amoving direction, for example, a lengthy one having a length of about 5to 10 cm. That is, the heating section is disposed on the side of oneface of a work (thin low-carbon steel), and a cooling section (firstcooling water supplying section) is provided on the side opposite to thework so as to correspond to a vicinity of a front portion of the heatingsection in the moving direction. Thereby, the vicinity of the frontportion of the heating section in the moving direction performs therapid heating treatment in the first process and the first cooling watersupplying section corresponding thereto performs the rapid coolingtreatment in the first process. The heating section and the firstcooling water supplying section move as a set thereof. Then, a site onthe work which has been subjected to the rapid heating and rapid coolingtreatments in the first process is rapidly reheated by the vicinity of arear portion of the heating section. Thereby, the rapid heatingtreatment in the second process is performed. Thereafter, a coolingsection (second cooling water supplying section) disposed to beseparated from the heating section by a predetermined distance rearwardin the moving direction rapidly cools the site on the work which hasbeen rapidly heated by the vicinity of the rear portion of the heatingsection to apply the rapid cooling treatment in the second process tothe site. Accordingly, when the lengthy heating section (coil) shown inFIG. 1C is used, the rapid heating in the first process and the secondprocess can be performed by one heating section (coil), so that ahigh-frequency induction heating apparatus which has a simple andinexpensive structure can be realized. Incidentally, as shown in FIG.1C, the second cooling water supplying section is disposed on the sameside as the heating section via the work, but it may be disposed on thesame side as the first cooling water supplying section. However, inorder to perform efficient rapid cooling, it is preferred that thesecond cooling water supplying section is disposed on the same side asthe heating section, as shown in FIG. 1C.

Incidentally, as the first heating section and the second heatingsection in the first process and the second process, a laser isprovided, so that each rapid heating treatment may be performed by laserheating.

Test Example 1

Heat treatment was applied to following respective Samples.

(1) Sample 1: a cold-rolled steel plate of an ordinary steel (SPCC)

-   -   Chemical Components (%): C=0.04, Si=0.02, Mn=0.26, P=0.011, and        S=0.006    -   Thickness: 0.5 mm, Width: 100 mm, and Length: 200 mm        (2) Sample 2: a cold-rolled steel plate of an ordinary steel        (SPCC)    -   Chemical Components (%): C=0.037, Si=0.004, Mn=0.19, P=0.013,        S=0.012, sol Al=0.015, Cu=0.02, Ni=0.02, and B=14 (PPM)    -   Thickness: 0.5 mm, Width: 100 mm, and Length: 200 mm        (3) Sample 3: a cold-rolled steel plate of an ordinary steel        (JSC440)    -   Chemical Components (%): C=0.12, Si=0.06, Mn=1.06, P=0.022, and        S=0.005    -   Thickness: 0.6 mm, Width: 100 mm, and Length: 200 mm

As the heat treatment apparatus, a high-frequency induction heatingapparatus provided with one set of the heating section and the coolingsection shown in FIG. 1A was used, where after the treatment in thefirst process was performed by the heating section and the cooling watersupplying section, each Sample was left down to room temperature, andthe treatment in the second process was then performed by the samehigh-frequency induction heating apparatus. As the treatment condition,the following two treatment conditions (A) and (B) were adopted.

Treatment Condition (A)

First Process

(1) Moving speed of the heating section and the cooling water supplyingsection: 800 mm/min.

(2) The coil of the heating section was adjusted to 120 A. A sample waspre-heated according to gradual temperature rising as the heatingsection came relatively close to the sample, but the sample was rapidlyheated from 400° C. to 1200° C. in about one second. Thereafter, thesample was held for about 2.5 seconds until the temperature thereofdropped to 1050° C., and it was then rapidly cooled to 200° C. or lowerin about 0.5 seconds by supplying cooling water from the cooling watersupplying section (a solid line in the first process shown in FIG. 2).

Second Process

(1) Moving speed of the heating section and the cooling water supplyingsection: 800 mm/min.

(2) After the sample dropped to room temperature, it was set in thehigh-frequency induction heating apparatus again. A current to be madeto flow in the coil of the heating section was adjusted to 100 A, andafter the sample was pre-heated to 400° C., it was rapidly heated up to900° C. in about 0.5 seconds. The sample was held for about 2.5 secondsuntil the temperature dropped to 800° C., it was then rapidly cooleddown to about 200° C. or lower in about 0.5 seconds by supplying coolingwater from the cooling water supplying section, and it was thereafterleft until the temperature reached room temperature (a solid line in thefirst process shown in FIG. 2).

Treatment Condition (B)

First Process

(1) Moving speed of the heating section and the cooling water supplyingsection: 800 mm/min.

(2) The coil of the heating section was adjusted to 120 A. A sample waspre-heated according to gradual temperature rising as the heatingsection came relatively close to the sample, but the sample was rapidlyheated from 400° C. to 1200° C. in about one second. Thereafter, thesample was held for about 2.5 seconds until the temperature thereofdropped to 1050° C., and it was then rapidly cooled to 200° C. or lowerin about 0.5 seconds by supplying cooling water from the cooling watersupplying section (a solid line in the first process shown in FIG. 2).

Second Process

(1) Moving speed of the heating section and the cooling water supplyingsection: 1000 mm/min.

(2) After the sample dropped to room temperature, it was set in thehigh-frequency induction heating apparatus again. A current to be madeto flow in the coil of the heating section was adjusted to 100 A, andafter the sample was pre-heated to 400° C., it was rapidly heated up to800° C. in about 0.5 seconds. The sample was held for about 2.5 secondsuntil the temperature dropped to 700° C., it was then rapidly cooleddown to about 200° C. or lower in about 0.5 seconds by supplying coolingwater from the cooling water supplying section, and it was thereafterleft until the temperature reached room temperature (a broken line inthe first process shown in FIG. 2).

FIG. 3A are electron microscope photographs of microstructures ofSamples 1 which were treated according to the treatment conditions (A)or (B) and which were observed by cutting near their central portions intheir longitudinal directions, and FIG. 3B are electron microscopephotographs of microstructures of the Samples 2 which were treatedaccording to the treatment conditions (A) or (B) and which were observedby cutting near their central portions in their longitudinal directions(incidentally, regarding the microstructures of the raw material statesof Samples 1 and Samples 2, see the column “Raw Material” in FIG. 6A).FIG. 3C are electron microscope photographs of microstructures ofSamples 3 which were treated according to the treatment conditions (A)or (B) and which were observed by cutting near their central portions intheir longitudinal directions.

From FIG. 3A, Sample 1 which was treated according to the treatmentcondition (A) was composed of a duplex grain size structure of a ferritestructure of fine grains having grain diameters of 1 μm to less than 5μm and a ferrite structure of grains having grain diameters of 5 to 30μm, where island-shaped martensites having a grain diameter of 30 μm orless were contained in the duplex grain size structure in an amount ofless than 5%. On the other hand, in the case of the treatment condition(B) where the moving speed was faster than that of the treatmentcondition (A) and the heating temperature in the second process waslower than that of the treatment condition (A), Sample 1 was composed ofa duplex grain size structure of a ferrite structure of fine grainshaving grain diameters of 1 μm to less than 5 μm and a ferrite structureof grains having grain diameters of 5 to 20 μm, so that crystal grainsof Sample 1 according to the treatment condition (A) were slightlylarger than those of Sample 1 according to the treatment condition (B).

In the case of FIG. 3B, Sample 2 which was treated according to thetreatment condition (A) contained island-shaped martensites having graindiameters of 30μ in an amount of about 20% in addition to a duplex grainsize structure of a ferrite structure of fine grains having graindiameters of 1 μm to less than 5 μm and a ferrite structure of grainshaving grain diameters of 5 to 30 μm. In the case of the treatmentcondition (B), Sample 2 was composed of a duplex grain size structure ofa ferrite structure of fine grains having grain diameters of 1 μm toless than 5 μm and a ferrite structure of grains having grain diametersof 5 to 20 μm.

Samples 3 contained C in an amount of 0.12% which was more than those ofSamples 1 and Samples 2. Accordingly, as shown in FIG. 3C, both Sampleswhich were treated according to the treatment condition (A) or (B)contained island-shaped martensites having grain diameters of 30 μm orless in addition to a duplex grain size structure of a ferrite structureof fine grains having grain diameters of 1 μm to less than 5 μm and aferrite structure of fine grains having grain diameters of 5 to 30 μm,where the island-shaped martensites were contained in an amount of about50 to 60%.

Test Example 2

The above Samples 1 was heat-treated by a high-frequency inductionheating apparatus provided with a heating section comprising a lengthycoil with a length of 6 cm shown in FIG. 1C and first and second coolingwater supplying sections. A treatment condition was as the following(C).

Treatment Condition (C)

First Process

(1) Moving speed of the heating section and the first and second coolingwater supplying sections: 800 mm/min.

(2) The coil of the heating section was adjusted to 120 A. A sample waspre-heated according to gradual temperature rising as the heatingsection came relatively close to the sample, but the sample was rapidlyheated from 400° C. to 1200° C. in about one second. Thereafter, thesample was held for about 2.5 seconds until the temperature thereofdropped to 1050° C., and it was then rapidly cooled to 200° C. or lowerin about 0.5 seconds by supplying cooling water from the cooling watersupplying section (a solid line in the first process shown in FIG. 4).

Second Process

(1) Moving speed of the heating section and the first and second coolingwater supplying sections: 1000 mm/min.

(2) A current to be made to flow in the coil of the heating section wasadjusted to 90 A and Sample 1 whose temperature dropped to about 200° C.was rapidly heated up to 800° C. in about 0.5 seconds by the rearportion of the heating section. Sample 1 was held for about 2.5 secondsuntil its temperature dropped to 700° C., it was then rapidly cooled to200° C. or less in about 0.5 seconds by supplying cooling water from thesecond cooling water supplying section, and thereafter it was left untilits temperature reached room temperature (a solid line in the secondprocess in FIG. 4).

FIG. 5 is an electron microscope photograph of a microstructure ofSample 1 which was treated according to the treatment condition (C) andwhich was observed by cutting near its central portion in itslongitudinal direction. From FIG. 5, Sample 1 which was treatedaccording to the treatment condition (C) included island-shapedmartensites having grain diameters of about 5 to 10 μm formed in anamount of about 20% in addition to a duplex grain size structure of aferrite structure of fine grains having grain diameters of 1 μm to lessthan 5 μm and a ferrite structure of grains having grain diameters of 5to 20 μm.

Comparative Example 1

As the heat treatment apparatus, the high-frequency induction heatingapparatus provided with one set of the heating section and the coolingwater supplying section shown in FIG. 1A was used, and heat treatmentwhere rapid heating and rapid cooling were only once performed wasperformed to Sample 1 (Comparative Sample 1) and Sample 2 (ComparativeSample 2).

Regarding the treatment condition, a case where after rapid heating wasperformed up to 1200° C. by the heating section (coil), rapid coolingwas performed by the cooling water supplying section (Heat Treatment 1)and a case where after rapid heating was performed up to 900° C. by theheating section (coil), rapid cooling was performed by the cooling watersupplying section (Heat Treatment 2) were tested. The condition of HeatTreatment 1 was aimed to produce a martensite structure while thecondition of Heat Treatment 2 was aimed to produce a duplex grain sizestructure or a duplex grain size structure including island-shapedmartensites. Electron microscope photographs of microstructures ofrespective Samples whose were observed by cutting at their centralportions in their longitudinal directions are shown in FIG. 6A to 6C.Incidentally, in these figures, “Raw Material” indicates microstructuresof Sample 1 and Sample 2 before heat treatment is performed thereto.

From FIG. 6A, both Sample 1 and Sample 2 in their raw material stateshave approximately-even ferrite structures of grains with graindiameters of 10 μm or less. Both Comparative Sample 1 and ComparativeSample 2 in their states of “Heat Treatment 1” shown in FIG. 6B havecoarse martensite structures of grains with grain diameter of 20 to 100μm. Comparative Sample 2 in its state of “Heat Treatment 2” shown inFIG. 6C has a duplex grain size structure of a ferrite structure of finegrains having grain diameters of 1 μm to less than 5 μm and a ferritestructure of grains having grain diameters of 5 to 30 μm. In the case ofComparative Sample 1, island-shaped martensites having a grain diameterof about 5 to 10 μm are formed in addition to a duplex grain sizestructure of a ferrite structure of fine grains having grain diametersof 1 μm to less than 5 μm and a ferrite structure of grains having graindiameters of 5 to 30 μm.

FIG. 7 is a graph where an average hardness (Hv) is represented on ahorizontal axis while fractal dimension of a grain diameter isrepresented on a vertical axis, and respective values of respectiveSamples 1, 2 and 3 of Test Example 1 and Test Example 2, and ComparativeSamples 1 and 2 of Comparative Example 1 are plotted. As apparent fromthis figure, in the case of Sample 1 and Sample 2, ones where the duplexgrain size structure was formed or island-shaped martensites were formedin the duplex grain size structure in both Test Examples 1 and 2 werehigher in fractal dimension than Comparative Samples 1 and 2 (Heattreatment 1) where martensite structures were formed in ComparativeExample 1. It was found that, regarding inclinations obtained byleast-square method, shown in FIG. 7, Test Examples 1 tended to behigher in fractal dimension than Comparative Examples 1 as a whole, andby performing rapid heating treatment and rapid cooling treatmentmultiple times like Test Example 1, even in ones with same duplex grainsize structure formed or with same island-shaped martensites formed in aduplex grain size structure, Samples 1 and 2—A treatment (treatmentaccording to the treatment condition (A)) and Samples 1 and 2—Btreatment (treatment according to the treatment condition (B)) could bemade finer in grain diameter and higher in toughness than ComparativeSamples 1 and 2 (Heat Treatment 2). Further, even in the case of Sample1—C treatment (treatment according to the treatment condition (C)) ofTest Example 2, though hardness became high, fractal dimension wasapproximately equal to the case of Comparative Sample 1 (Heat Treatment2) formed with the duplex grain size structure.

Further, very high hardness was obtained in Sample 3 of Test Example 1.This is because a dispersion percentage of island-shaped martensites ishigh, and Sample 3 of Test Example 1 is inferior to Samples 1 and 2—Atreatment and Samples 1 and 2—B treatment in toughness. However, whencompared with Comparative Samples 1 and 2 (Heat Treatment 1), it isfound that Sample 3 of Test Example 1 maintained hardness which was notinferior to that of the case having the martensite structure and shownin FIG. 6B, while it became high in fractal dimension, so that it couldbe increased in toughness while it maintained hardness higher than thoseof Comparative Samples 1 and 2 (Heat Treatment 1). However, when Ccontent is more than that of Sample 3 of Test Example 1, there is apossibility that the toughness is further inferior, so that it is moredesirable that the C content is set to 0.12% or less.

Incidentally, when high strength is achieved by refining crystal grainsin a metal structure, the fractal dimension is largely raised accordingto Law of Hall-Petch as shown by arrow X as compared with the state ofraw materials shown in FIG. 7, but the fact that high strength does notdepend on refinement of crystal grains in the case of the technique ofthe present invention utilizing ultra-rapid heating and ultra-rapidcooling is also understood from such a fact that a rising percentage ofa value of a fractal dimension showed a small tendency.

FIG. 8 is a graph where a breaking elongation (%) is represented on ahorizontal axis while fractal dimension of a grain diameter isrepresented on a vertical axis, and respective values of respectiveSamples 1 and 2 of Test Example 1 and Test Example 2, and ComparativeSamples 1 and 2 of Comparative Example 1. For example, “Sample 1—Atreatment” indicates one where island-shaped martensites are containedin the above-described duplex grain size structure in an amount of lessthan 5% and whose breaking elongation is 18.16%, while “Sample 2—Atreatment” indicates one which is composed of the above-described duplexgrain size structure and whose breaking elongation is 20.44%. Thebreaking elongation which is one of indexes of the toughness tends tobecome large according to the increase of the fractal dimension, so thata correlation between the above-described fractal dimension andtoughness became apparent. Then, it was also found from FIG. 8 thatSamples 1 and 2 of the present invention which were subjected to rapidheating treatment and rapid cooling treatment multiple times couldelevate their toughness as compared with Comparative Samples 1 and 2which were subjected to rapid heating treatment and rapid coolingtreatment only once.

Text Example 3 Bending Test

Three kinds of samples having the same chemical components as those ofthe cold-rolled steel plate of the ordinary steel of Sample 1 and whosethicknesses were 0.5 mm, 0.8 mm, and 1.0 mm, respectively, wereheat-treated such that the heat treatment covers their ranges of a widthof 30 mm and a length of 100 mm (see FIG. 9A). In the heat treatment,each treatment included in the first process and the second process wasperformed according to the above “Treatment Condition (A)”.

As shown in FIG. 9B, each of the above-described Samples was set on thesupporting stand supporting the vicinities of both ends thereof, and aload was imparted on a central portion of the heat-treated range of eachSample in a longitudinal direction thereof at a loading rate of 10mm/min by a crosshead. Respective ones of non-heated ones (representedas “Raw Material” in FIG. 10) and heat-treated ones (“Heat Treatment” inFIG. 10) of all Samples were tested. The test result is shown in FIG.10.

As apparent from FIG. 10, regarding a reaction force due to deflectionof a beam caused by a bending moment at a transition point from anelastic region to a plastic region in a bending property, Sample with athickness of 0.5 mm which was heat-treated is about twice the Samplebefore heat-treated, and Samples with a thickness of 0.8 mm and with athickness of 1.0 mm which were heat-treated are about 2.5 times thosebefore heat-treated. Accordingly, by using Sample with a thickness of0.5 mm which was heat-treated instead of a raw material with a thicknessof 0.8 mm or using Sample with a thickness of 0.8 mm which washeat-treated instead of a raw material with a thickness of 1.0 mm,contribution to weight reduction of a seat frame or the like can beachieved.

Test Example 4 Tensile Test

Tests were performed by grasping end portions of samples with a lengthof 150 mm and a width of 30 mm in their longitudinal directions by achuck. The samples were Samples 1 with a thickness of 0.5 mm and with athickness of 0.8 mm which were used in the above-described bending testsand Sample 2 with a thickness of 0.5 mm. The result is shown in FIG. 11.In FIG. 11, “Heat treatment—A (Sample 1)” and “Heat Treatment—A (Sample2)” were heat-treated according to the above-described treatmentcondition (A) of Test Example 1, where the microstructure was the duplexgrain size structure or it was the duplex grain size structure formedtherein with island-shaped martensites. “Heat treatment 1” is a samplewhich was heat-treated according to the above-described “Heat Treatment1” of Comparative Example 1 and which resulted in martensite structure.

As a result, the yield point (proof stress) of the sample formed withthe martensite structure of Heat Treatment 1 in Comparative Example 1 ishigh but the breaking elongation thereof is low. On the other hand, theyield points (proof stresses) of “Heat treatment—A (Sample 1)” and “HeatTreatment—A (Sample 2)”, when having a thickness of 0.5 mm, were abouttwice that of a raw material before heat-treated and they were lowerthan that of one formed with a martensite structure, but the breakingelongations thereof were at least three times that of the one formedwith a martensite structure. The yield points (proof stresses) of “Heattreatment—A (Sample 1)” and “Heat Treatment—A (Sample 2)”, when having athickness of 0.8 mm, were about 2.5 times that of a raw material beforeheat-treated, but the breaking elongation thereof were about twice thatof one formed with a martensite structure.

Test Example 5

A steel pipe made of carbon steel for machine or structure (STKM-13C)with a diameter of 12 mm, a thickness of 1.0 mm, and C content of 0.08%was heat-treated while being rotated at a rotation speed of 400 rpm.Regarding the case where the heat treatments in the first process andthe second process were performed by the high-frequency inductionheating apparatus shown in FIG. 1A (represented as “two-stage heattreatment” in FIG. 12) and the case where only the heat treatment in thefirst process was performed (represented as “one-stage heat treatment”in FIG. 12), tensile tests were performed and compared with each other.The result is shown in FIG. 12.

As apparent from FIG. 12, samples which were subjected to the two-stageheat treatment of the present invention became about at least twice thatof a raw material regarding the yield point (proof stress) and they hadbreaking elongation about twice that of a sample which was subjected tothe one-stage heat treatment. Incidentally, since the “Raw Material” inFIG. 12 was not attached with an elongation meter, a rising of a graphthereof was different from those of the other samples which wereheat-treated, but the breaking elongation thereof was corrected fromactual measured values. Further, in graphs of samples which wereheat-treated, the reason why loads applied to the samples droppedhalfway was because a measuring machine was stopped and the elongationmeter was detached from the samples halfway since while the sample wasbeing attached with a measuring tool, it could not be measured untilbreaking took place.

From the above, it was found that all of the hardness, the yield point(proof stress), the tensile strength, the reaction force due todeflection of a beam caused by a bending moment, and the breakingelongation of a steel where the microstructure which was subjected tothe heat treatment of the present invention was a duplex grain sizestructure or a duplex grain size structure formed with island-shapedmartensites, namely, a steel which was subjected to the rapid heatingand rapid cooling treatments in the first process and the second processwere maintained in high level, and a steel having high strength and hightoughness (high ductility) could be obtained while it was obtained byheat-treating a commercially-available ordinary steel.

1-19. (canceled)
 20. A heat treatment apparatus which uses a thinlow-carbon steel which is an ordinary steel which has been worked tohave a thickness of 1.2 mm or less as the steel raw material which is areception material of heat treatment and which is used to manufacture ahigh-strength and high-toughness thin steel by a first process ofrapidly cooling the thin low-carbon steel after rapid heating thereof toobtain a martensite structure and a second process of rapidly coolingthe thin low-carbon steel which have been subjected to the first processafter rapidly reheating the same to a temperature lower than atemperature at a rapid heating time in the first process, wherein a worksupporting section which supports the thin low-carbon steel which is atarget to be treated, a first heating section which performs the rapidheating treatment in the first process, a first cooling section whichperforms the rapid cooling treatment in the first process, a secondheating section which performs the rapid heating treatment in the secondprocess, and a second cooling section which performs the rapid coolingtreatment in the second process are sequentially arranged; and the firstheating section, the first cooling section, the second heating section,and the second cooling section are provided so as to be movable relativeto the work supporting section; wherein one heating section having apredetermined length extending in a moving direction, the first coolingsection arranged on the side opposite to the heating section via thework, and the second cooling section arranged on the same side as theheating section or on the same side as the first cooling section via thework to be separated from the heating section or the first coolingsection by a predetermined distance rearward in the moving direction areprovided, and the heating section has such a length that a vicinity of afront portion thereof corresponds to the first cooling section and avicinity of a rear portion thereof extends rearward in the movingdirection beyond the first cooling section; and the heating section isconfigured to have two functions such that the vicinity of the frontportion of the heating section has a function of the first heatingsection which performs the rapid heating in the first process and thevicinity of the rear portion of the heating section has a function ofthe second heating section which performs the rapid heating in thesecond process.
 21. The heat treatment apparatus according to claim 20,wherein the second cooling section is disposed on the same side as theheating section.
 22. The heat treatment apparatus according to claim 20,wherein the work supporting section is rotatably provided in asupporting state of the thin low-carbon steel.
 23. The heat treatmentapparatus according to claim 20, wherein each of the heating sectionsincludes a coil performing high-frequency induction heating.
 24. Theheat treatment apparatus according to claim 20, wherein each of theheating sections is provided with a laser performing laser heating. 25.The heat treatment apparatus according to claim 20, wherein each of thefirst cooling section and the second cooling section is a cooling watersupplying section supplying cooling water.
 26. The heat treatmentapparatus according to claim 20, wherein the first process includes astep of rapidly heating the thin low-carbon steel up to a temperature of1000° C. or higher at a rate of 300° C./second or more and a step ofrapidly cooling the thin low-carbon steel at a rate of 300° C./second ormore after the thin low-carbon steel is held at a temperature of 900° C.or higher within ten seconds; and the second process includes a step ofrapidly heating the thin low-carbon steel up to a temperature of 700° C.or higher at a rate of 300° C./second or more after the cooling in thefirst process and a step of rapidly cooling the thin low-carbon steel ata rate of 300° C./second or more after the thin low-carbon steel is heldat a temperature of 600° C. or higher within ten seconds.
 27. The heattreatment apparatus according to claim 26, wherein rapid heating isperformed up to a temperature in a range of 1000° C. to 1250° C. at therapid heating step in the first process and rapid heating is performedup to a temperature in a range of 750° C. to 1050° C. at the rapidheating step in the second process.
 28. The heat treatment apparatusaccording to claim 26, wherein a holding time before the rapid coolingafter the rapid heating in the first process is set within five seconds,and the holding time before the rapid cooling after the rapid heating inthe second process is set within five seconds.
 29. The heat treatmentapparatus according to claim 26, wherein C content of the thinlow-carbon steel is in a range of 0.01 to 0.12% by mass % and the restthereof is composed of iron and inevitable impurities.
 30. The heattreatment apparatus according to claim 26, wherein the steel rawmaterial which is a reception material of heat treatment is a thinlow-carbon steel with a thickness of 1.0 mm or less.
 31. The heattreatment apparatus according to claim 26, wherein the steel rawmaterial which is a reception material of heat treatment is a thinlow-carbon steel with a thickness of 0.8 mm or less
 32. The heattreatment apparatus according to claim 26, wherein the steel rawmaterial which is a reception material of heat treatment is a thinlow-carbon steel with a thickness of 0.5 mm or less.